The present invention relates to a semiconductor device fabrication method.
Recently, to increase the degree of integration and the operating speed and reduce the power consumption and the fabrication cost, micropatterning of LSIs has advanced so that the minimum processing dimension (e.g., the gate length of a transistor) is nearly 0.1 μm. This LSI micropatterning is expected to advance in the future until the minimum processing dimension becomes 0.1 μm or less. For example, logic devices in which the gate length of a transistor is decreased to about 30 nm are developed.
To micropattern an element such as a transistor, it is important to micropattern an element isolation region which occupies more than half the element area. Recently, an STI (Shallow Trench Isolation) method is used as a method of forming this element isolation region. In this STI method, an element isolation trench is formed by etching the surface portion of a semiconductor substrate, and an element isolation region (i.e., an element isolation insulating film) is formed by burying an insulating film in this element isolation trench. By the use of this STI method, the width of an element isolation region reaches about 70 to 90 nm smaller than 0.1 μm.
Also, in a memory requiring a high degree of integration, the widths of both an element formation region (active area) in which a transistor and the like are formed and an element isolation region reach about 70 to 90 nm smaller than 0.1 μm. In a memory like this, micropatterning of an element isolation region is important.
On the other hand, micropatterning of elements makes the formation of an element isolation region difficult. Separation between adjacent elements is determined by the effective distance between the adjacent elements, i.e., the shortest distance (the depth of an element isolation trench×2+the width of the element isolation trench) when a circuit is made around an element isolation region.
Even when a device is micropatterned, therefore, the effective distance, i.e., the depth of an element isolation trench must be maintained in order not to deteriorate the insulation properties of adjacent elements. Since, however, the width of an element isolation trench is decreased by micropatterning, the aspect ratio (the depth of the element isolation trench/the width of the element isolation trench) of the element isolation trench increases as micropatterning advances. This makes it difficult to bury an insulating film in the element isolation trench.
As a method of burying an insulating film in an element isolation trench like this, high-density plasma (HDP) CVD is used. When a silicon oxide film as an insulating film is to be buried in an element isolation trench by using this high-density plasma CVD, the aspect ratio is 3 or more if the minimum processing dimension is 0.1 μm or less. This results in the inconvenience that voids (unfilled portions) readily form in the insulating film buried in the element isolation trench.
As a method of burying an insulating film in a micropatterned element isolation trench, therefore, it is possible to form and bury an SOG (Spin On Glass) film by spin coating (by which a semiconductor substrate is coated with a predetermined solution while being rotated).
It is also possible to form and bury a silicon oxide film by reacting TEOS (Tetraethoxysilane) gas having fluidity with O3 (ozone) gas.
In still another method, a silicon oxide film is buried in an element isolation trench by using high-density plasma CVD, and a silicon oxide film formed by reacting TEOS gas with O3 (ozone) gas is buried in portions not filled by high-density plasma CVD.
Recently, a semiconductor substrate is coated with a silazane perhydride polymer solution so as to fill an element isolation trench formed in the substrate, and oxidation is performed in a steam ambient to form a silicon oxide film as an element isolation insulating film (e.g., references 1 and 2).
More specifically, a silazane perhydride polymer solution is prepared by dispersing a silazane perhydride polymer ((SiH2NH)n) in a solvent such as xylene (C6H4(CH3)2) or dibutylether ((C4H9)20).
Then, the surface of a semiconductor substrate is coated with this silazane perhydride polymer solution by spin coating so as to fill an element isolation trench formed in the substrate. A predetermined heat treatment is performed on this coated silazane perhydride polymer solution to volatilize the solvent in it, thereby forming a polysilazane film. After that, the polysilazane film is oxidized to form a silicon oxide (SiO2) film as an element isolation insulating film.
In the polysilazane film formed by volatilizing the solvent in the silazane perhydride polymer solution, carbon (C) contained in the solvent such as xylene (C6H4(CH3)2) or dibutylether ((C4H9)20) remains as an impurity.
Accordingly, to form a silicon oxide (SiO2) film having high film quality, it is necessary to remove carbon (C) as an impurity by increasing the oxidation amount in the oxidation process. However, if the oxidation amount is increased while a silicon oxide film serving as a gate insulating film and a polysilicon film serving as a gate electrode are formed in an element formation region (active area), these silicon oxide film and polysilicon film oxidize. As a consequence, the electrical characteristics and reliability of the transistor deteriorate.
On the other hand, if the oxidation amount is decreased to suppress oxidation in this element formation region, an impurity such as carbon (C) remains in the silicon oxide (SiO2) film and functions as positive fixed electric charge. Consequently, the electrical characteristics and reliability of the transistor deteriorate in this case as well.
References related to the element isolation insulating film formation method are as follows.
Reference 1: Japanese Patent Laid-Open No. 2004-179614
Reference 2: Japanese Patent Laid-Open No. 2002-367980
It is an object of the present invention to provide a semiconductor device fabrication method capable of suppressing deterioration of the electrical characteristics and reliability of a transistor.